As mentioned, TMo and Sprint also own/lease towers. The big 4 are all real full network operators. They all have roaming agreements with other smaller carriers

As mentioned, TMo and Sprint also own/lease towers. The big 4 are all real full network operators. They all have roaming agreements with other smaller carriers in areas they don't serve, but TMo and Sprint are not MVNOs, however they both OWN MVNOs.